Patterns of Alpha-band Power Reflect Prioritization of the Most Recently Encoded Location in a Working Memory Sequence.

Space and time play a critical role in working memory formation and maintenance relative to other object features. Particularly, covert spatial attention spontaneously tracks object locations in working memory and is thought to enable feature binding at those location markers. However, whether object locations are attended concurrently as multiple objects are presented in a sequence remains a topic of debate. Here, we investigated whether spatial attention contributes to sustained active memory maintenance when items are separated in both space and time. We report two experiments that differed in whether color or location was the tested feature. In each experiment, we measured EEG alpha-band power while participants memorized the target feature of two sequentially presented colored dots. We then applied an inverted encoding model to the topography of oscillatory alpha-band power (8-12 Hz) on the scalp to measure how long the location of each item was attended. In Experiment 1, participants performed a delayed color estimation task, and we found that patterns of alpha-band power tracked the location of the most recently presented item. In Experiment 2, participants performed a delayed spatial estimation task, and we found that patterns of alpha-band power tracked the location of Item 1 for longer than observed in Experiment 1. However, only the most recently presented item (Item 2) sustained until the end of the delay. Taken together, our results show that, although both items need to be remembered, covert spatial attention prioritizes items one at a time when they are displayed in a sequence.